There has been known that there is a sintered material mainly composed of silicon nitride and containing rare earth elements, oxides of aluminum or magnesium or a compound oxides thereof as sintering aid, which material is superior in mechanical strength as a structural material.
Heretofore, as this type of the silicon nitride sintered material, there have been known a product obtained on mixing 1 to 7 wt % of powders of a sintering aid of Yb.sub.2 O.sub.3 /MgAl.sub.2 O.sub.4 =5/2 to 2/2 to 99 to 93 wt % of powders of silicon nitride and sintering the resulting mixture (JP-Patent Kokai Publication JP-A-3-164472), a product containing 3 to 6 wt % of Yb.sub.2 O.sub.3 and 0.01 to 0.03 wt % of MgAl.sub.2 O.sub.4 to powders of silicon nitride and composed of granular crystals and columnar crystals with a mean short diameter of 0.2 to 0.6 .mu.m (JP Patent Kokai Publication JP-A-4-202060) and a product containing dispersed grains of TiO.sub.2 in addition to columnar crystals of Si.sub.3 N.sub.4 and a grain boundary phase formed by sintering aids such as Y.sub.2 O.sub.3, MgO and/or Al.sub.2 O.sub.3 etc., with the short diameter and the long diameter of the columnar crystal being not more than 0.3 .mu.m and not more than 5 .mu.m, respectively (JP Patent Kokai Publication JP-A-6-305837).
However, according to eager investigation of the inventors of the present invention, there is found a following problem in the art. Namely, above conventional techniques are concerned only with improving the mechanical strength, without being concerned with hardness. In addition, no reference is made to applicability of the material as a wear resistant member (slide member) or a jig for which both strength and hardness are required. Tracing experiments, conducted by the present inventors, have revealed that the material is not sufficient in hardness for application to a rolling roll, squeeze roll, bearing ball, drawing die or punching dies.